The evolution of mass-consumer wireless communication products balances a compromise between the demands of the users for ever higher data rate and quality of service and the demands of the service-providers and manufacturers for cost reduction, which ultimately leads to products that are physically possible to produce and operate efficiently that aim to satisfy user demands. The high data rate wireless communication products need large channel capacity, which can be achieved by both wide signal bandwidths and maintenance of a high signal-to-noise (SNR) ratio. A power-efficient approach adopted by a number of the evolving wireless communication standards is to choose SNR levels that do not lead to a too large power consumption, whilst utilizing high radio frequency (RF) carriers that make available wide signal bandwidths. However, it is known that the propagation losses increase at high RF, which can be compensated for by increasing the antenna gain.
As a consequence, phased array transceiver architectures have been developed for new mass-consumer wireless communication applications, which can effectively increase the antenna gain. The phased-array concept uses a-priori information about the location of the communicating party to form a directed (rather than omni-directional) beam between the transmitting and receiving communication units or the radar unit and a target object. Thus, communication energy is used where needed, which effectively increases the directivity of the antenna. Such a spatial selectivity is moreover very much required (or at least desired) for the attenuation of unwanted signals, i.e. blockers and interferers, in order to guarantee a particular quality of service. With such approaches, the desired signal is constructively combined, whilst the spatially adjacent signal interferences are destructively combined.
A number of current communication applications employ multiple receivers and transmitters (the combination of which is referred to as transceivers (TRx)), such as some mobile communications and commercial automotive radar sensors. Typically, multiple receivers and transmitters are implemented as a phased array antenna system, in order to improve the output power, receiver sensitivity and angular resolution for directed wirelessly transmitted and received signals. A microcontroller unit (MCU) performs digital control of the transceiver circuits and digital signal processing of the digitized data in order to output processed data. Such multiple-path architectures with multiple distinct outputs are typically referred to as multiple-in multiple-out (MIMO), whereas multiple-path architectures with a single distinct output (after summation/processing) are typically referred to as multiple-in single-out (MISO).
Another array-based approach to increase the overall communication capacity is to use MIMO communication links, where multiple independent streams are being used between the parties. Thus, a single base-station, for example, can serve multiple customers, which increases the overall utilization efficiency of the available channel capacity, but also significantly reduces the costs for the service providers. However, such an approach requires an array of ADCs, which thereafter requires all prohibitively large amounts of power and renders itself practically unrealistic when considering the current state-of-the-art of IC technologies. Unfortunately, the phased array receivers are practically bounded by the power dissipation, and hence their power efficiency is important to achieving a high performance.
FIG. 1 illustrates various block diagrams of known MIMO or MISO receiver architectures. Each of the receiver architectures include standard components, such as a plurality (or an array) of receiver antennas 102, each coupled to a respective low noise amplifier (LNA) 104 and thereafter a down-mixer 110 that receives a local oscillator signal 112. The output of the down-mixer is input to a transimpedance amplifier (TIA) 114 and then an analog to digital converter (ADC) 116 to produce a digital output of the wireless signal received across the plurality (or array) of receiver antennas 102. Each of the receiver architectures includes a phase shifting component 106.
The first three receiver architectures implement analog beam steering using a phased array summation of the signal as early as possible to implement spatial selectivity, which is a power-efficient approach (to be checked). A first example receiver architecture 100 illustrates a known technique whereby beam steering of the wireless signal received across the plurality (or array) of receiver antennas 102 is performed by a RF phase shifting component 106 in the RF domain, and typically immediately after the LNA 104. In this example, the outputs of the respective phase shifted signals are summed in summing junction 108, before the combined RF signal is down-converted by down-mixer 108. The first example receiver 100 therefore requires a number of RF phase shifting components 106, which are both bulky and costly.
A second example receiver architecture 130 illustrates a known technique whereby beam steering of the wireless signal received across the plurality (or array) of receiver antennas 102 is also performed in the RF domain using LO phase shifting components 106 in order to adjust the phase of the received RF signal following down-conversion by the down-mixer 110. In this example, the outputs of the respective phase shifted and down-converted signals are summed in summing junction 108, before the analog baseband signal is input to a TIA 114 and then an ADC 116 to produce a digital output of the wireless signal received across the plurality (or array) of receiver antennas 102 and respectively phase shifted. However, the second example receiver 130 therefore also requires a number of LO phase shifting components 106, which are both bulky and.
A third example receiver architecture 160 illustrates a known technique whereby beam steering of the wireless signal received across the plurality (or array) of receiver antennas 102 is performed in an intermediate frequency (IF) or baseband domain using IF or analog baseband phase shifting components 106 to adjust the phase of the received and down-converted RF signal. In this example, the outputs of the respective down-mixers 110 are respectively phase-shifted and summed in summing junction 108, before the analog baseband signal is input to a TIA 114 and then an ADC 116 to produce a digital output of the wireless signal received across the plurality (or array) of receiver antennas 102. However, the third example receiver 160 requires accurate analog delays to provide receiver beam steering, which is complex and difficult to readily implement.
The analog beam steering approach is theoretically able to remove (or substantially remove) any unwanted signals based on spatial selectivity and, hence, may relax the dynamic range requirements for all electronic building blocks beyond the phased-array signal summation, e.g. mixers and ADC(s). However, such an approach that requires receiving multiple streams is problematic, since the multiple instantiations of the receiver for each data path in, say, a communication base-station, results in a too large IC footprint area and power consumption. Additionally, the cost is increased. Furthermore, for the first and third examples where the analog phase shifters (implementing beam steering) are located in the signal path, the receiver architecture poses tough requirements on their noise and linearity performance, thereby resulting in high power consumption and low efficiency.
Alternatively, digital beam-steering can serve multiple streams by realizing the phased-array signal summation in the digital domain, i.e. after the ADCs. A fourth example receiver architecture 190 illustrates a known technique whereby beam steering of the wireless signal received across the plurality (or array) of receiver antennas 102 is performed in the digital domain whereby phase shifting is performed after respective analog-to-digital conversion of the multiple received signals. Here, no noise or non-linearity is added to the signal. However, the fourth example receiver 190 requires a number of accurate ADCs to provide receiver beam steering, which is complex and space consuming in an IC implementation. Furthermore, the spatial selectivity, and hence the interferer rejection, also takes place only in the digital domain. The design requirements for the electronic components in the receiver chain are significantly increased in comparison with the analog beam steering approaches. A larger dynamic range is also required for filters, mixers, ADCs, which increases so much so that the increased power consumption makes this approach practically infeasible. In addition, for each antenna, a full receiver path with LNA, mixers, TIAs and ADC is required for each received signal.
Thus, a mechanism is needed to provide improved beam steering for communication units (with transmitters and/or receivers) that use multiple antennas or an antenna array.
Referring now to FIG. 2, example diagrams 200, 250 of known sigma-delta modulators (SDMs) with nth order continuous-time loop filters (H1(s)-Hn(s) 208, 258), core ADCs 216, 266 and distributed feedback digital-to-analog converters (DACs) (b1-bn) 212, 262 is illustrated. The second example diagram 250 illustrates a known sigma-delta modulator with signal-feedforward coefficients. The signal transfer function (STF) and noise transfer function (NTF) are determined by the choice of continuous-time loop filers (H1(s)-Hn(s) 208, 258) as well as the gains of the digital-to-analog converters (DACs) (b1-bn) 212, 262.
Sigma-delta modulation (SDM) is a method for encoding analog signals in an ADC, e.g. converting analog input signals 202, 252 into digital signals, e.g. output 210, 260. All SDM structures realize the shaping of noise with an error minimizing feedback loop in which the analog input signal ‘x’ 202, 252 is compared with an analog version of the quantized digital output signal ‘y’, 210, 260. The difference between these two signals is frequency weighed with the continuous-time loop filers (H1(s)-Hn(s) 208, 258). Differences between the input signal x 202, 252 and output signal ‘y’210, 260 that fall in the signal band are passed to the output without attenuation, out-of-band differences are suppressed by the digital continuous-time loop filters (H1(s)-Hn(s) 208, 258). The result of the weighing is passed to the quantizer, which generates the next output value ‘y’. The output ‘y’ is also fed back to the input, to be used in the next comparison. A practical loop-filter realization consists of additional points, integrator sections, feed-forward coefficients ai 204, 254 and feed-back coefficients bi 212, 262 and multiple summing junctions 206, 256. The result of this strategy is a close match of input signal and quantized output in the pass-band of the filter, and shaping of the quantization errors.
A temporary use of a lower-resolution signal in an SDM simplifies circuit design and improves efficiency. An SDM may also be used to convert high bit-count, low-frequency digital signals into lower bit-count, higher-frequency digital signals, for example as part of the process to convert digital signals into an analog form as part of a digital-to-analog converter (DAC). Hence, SDMs also find applicability in a large number of transmitter paths, as well as receiver paths.